In the most recent combined analyses of the data for workers in
several countries exposed to low-level external radiation, Cardis
et al. [36,25] present an excellent discussion of the
importance of and difficulties encountered in studies of occupational
effects of external radiation. The subgroup of workers from Oak Ridge
that are in these combined studies are referred to as the ORNL
subpopulation and correspond to X-10 only workers in this report.
Their estimate of ERR per Sv for the ORNL subpopulation for all cancer
excluding leukemia is 1.66 with 90% CI = (.04, 4.4) and for leukemia
excluding CLL their estimate is -1.06 90% CI = (< 0, 4.8) ---see
[25] Table V). These results are consistent with the results
presented here for the larger X-10/Y-12 subcohort of Oak Ridge workers
that were potentially exposed to external ionizing radiation. Cardis
et al ---see [25] Table IV---give estimates of ERR per
Sv for all cancer for their combined data of -0.02 90% CI: (-.34,
.35); for leukemia the estimate is 1.55 with 90%CI: (-0.21, 4.7).

A recent high dose study by Pierce et al.[37] presented
risk estimates based on mortality in the Atomic Bomb Survivors through
1990 (see Table AI). Their excess relative risk estimates for all
cancers [0.37 per Sv 90% CI= (0.31,0.44)] and lung cancer [0.42 per
Sv 90% CI= (0.24,0.63)] are compatible with those derived here.
Failure of this study to detect a significant dose-response for
leukemia may be due to a lack of power. Another possibility is the
presence of an unidentified toxic chemical that is only present at
X-10 (e.g. some type of solvent) that is independent of, or
negatively correlated with, external dose, and causes leukemia. This
would also be a possible explanation of the increase in leukemia risk
at X-10 relative to U.S rates and other Oak Ridge facilities.

The combined risk estimates derived from the present study are
generally comparable to the positive dose response estimates in Pierce
et al. for those categories where there were larger numbers of
deaths. However, this study also suggests that there are differences
in dose response between worker cohorts in the same location followed
over about the same time period, and that these differences are
associated with measurement issues. Specifically, dose response
associations are stronger for X-10 workers, the population with more
complete external radiation monitoring. Differences in dose response
between X-10 and other workers are larger under longer latency
assumptions that depend more on exposure classification based on
radiation data from that historical period when monitoring differences
between populations were greatest. This observation suggests that
investigations of external radiation effects in combined facility
studies should be sensitive to differences in other exposures and in
radiation measurement characteristics between the cohorts.